Synchronous optical networks (SONETs), which provide very high data rate fiber optic links for communications, require low phase noise local oscillators for clock recovery. Phase noise, and the resulting effect of signal edge jitter in the local oscillator output, limits the clock speed or pulse rate for clock recovery by contributing to the required pulse width or duration for accurate operation. Additionally, the local oscillator employed in such applications should be frequency-tunable, allowing the local oscillator to be set or adjusted to a specific frequency to, for example, track frequency variations in the received clock signal. However, maintaining low-phase noise and providing significant tune range for a local oscillator have proven to be conflicting objectives.
Local oscillators are often constructed by placing a device within the feedback loop of an amplifier to cause the amplifier output to oscillate. Crystal oscillators are commonly employed for this purpose, but introduce substantial phase noise and therefore constrain use of the oscillator to lower frequency applications. While the output of a low frequency crystal stabilized oscillator may be multiplied up to a higher frequency or utilized with a frequency synthesizer, the phase noise is also multiplied up or otherwise translated proportionally into the output signal.
Surface acoustic wave (SAW) devices, when utilized in place of a crystal as a frequency reference in an amplifier-based oscillator, intrinsically have a high frequency response quality factor (Q) and therefor automatically provide low phase noise in the oscillator output. However, since SAW oscillators do not have the frequency accuracy of a crystal oscillator, the oscillator must be made frequency tunable to be adjusted to the precise frequency of interest. Typically this is accomplished through an adjustable phase shifter within the loop, with a substantial increase in phase noise.
U.S. Pat. No. 4,760,352 discloses a coupled resonator phase shift oscillator formed by connecting a SAW coupled (two pole) resonator within the feedback loop of an amplifier, and also describes earlier oscillators which employ a (SAW) delay line within the feedback network. However, both structures introduce approximately 180° phase shift across the passband, requiring a 180° phase shifter within the loop, which is difficult to build in a manner which is easily manufacturable. Moreover, a high Q circuit by definition exhibits a narrow passband within the frequency response curve, limiting the tune range of the oscillator to a small range of frequencies.
One approach to increasing the tune range of an oscillator employing a SAW resonator is disclosed in U.S. Pat. No. 6,239,664. Within a relatively narrow frequency range, the SAW resonator has an equivalent circuit similar to that of a bulk crystal, as shown in FIG. 4. Within that frequency range, the equivalent circuit 401 of the SAW resonator includes a series resonator comprising an inductance LM, a capacitance CM and a resistance RM all connected in series, with a shunt capacitance CO in parallel with the series resonator and formed by the internal parasitic and package capacitance of the SAW resonator. To make the SAW resonator tunable, an inductor LO sized to effectively tune out capacitance CO is connected in parallel with the SAW resonator 401 and a variable tuning capacitance CTUNE, such as a varactor diode, is connected in series with the SAW resonator 401. As the capacitance of tuning capacitance CTUNE decreases, the center frequency for the passband of the single port resonator circuit 400 increases.
The frequency range across which the SAW resonator 401 has the equivalent circuit shown, while relatively small, is both larger than the passband of the SAW and large enough to provide the tuning capability required. The disadvantage of the single port SAW resonator circuit 400 is that the circuit 400 has one or more secondary responses 500, as shown in FIG. 5, because the shunt inductor LO resonates with the tuning capacitance CTUNE at another frequency (other than the desired passband center frequency). Accordingly, U.S. Pat. No. 6,239,664 discloses (not shown in FIG. 4) an additional inductance and capacitance in conjunction with an amplifier stage to effectively eliminate any secondary responses. Within the passband of the SAW resonator, the SAW resonator circuit 400 provides a low impedance path to ground for the amplifier, forming a Colpitts oscillator. However, the amplifier must present a negative resistance which is greater than the resistance of the tuned SAW resonator circuit 400 in order for the circuit to oscillate.
Due to the additional tuning requirements necessary to tune out the secondary response(s), the SAW resonator oscillator disclosed in U.S. Pat. No. 6,239,664 is not easily manufactured reliably in quantity, and spurious responses are seen during manufacturing. Moreover, the structure is complex, with the tuning of the inductive coils and the values of capacitances, including the parasitic capacitances, being critical. Finally, the structure is large, requiring a dual in-line package for a practical implementation.
There is therefore a need in the art for a local oscillator employing a SAW resonator for low phase noise while providing an acceptable tune range.